#!/usr/bin/perl
use strict;
use warnings;

print common_substr('ABCD', 4, 'BCD', 3, 2);
{
    my %seen;
    sub common_substr {
        my ($str1, $len_str1, $str2, $len_str2, $len_subs) = @_;
        my $temp1 = exists $seen{$len_str1}{$len_subs}
                  ? $seen{$len_str1}{$len_subs}
                  : ($seen{$len_str1}{$len_subs} = ("a${len_subs}X" . ($len_subs - 1)) x ($len_str1 - $len_subs + 1));
        my %substr;
        @substr{ unpack($temp1, $str1) } = ();
        my $temp2 = exists $seen{$len_str2}{$len_subs}
                  ? $seen{$len_str2}{$len_subs}
                  : ($seen{$len_str2}{$len_subs} = ("a${len_subs}X" . ($len_subs - 1)) x ($len_str2 - $len_subs + 1));
        my $count = keys %substr;
        delete @substr{ unpack($temp1, $str2) };
        return $count - keys %substr;
    }
}